In urban daily commute, especially during the peak time, people are frequently stuck in heavy traffic. In order to cut the emission of carbon dioxide of cars as well as reduce other pollutions while a car stops in traffic congestion or waits at a traffic light, most car manufacturers are introducing a start-stop engine function. With the start-stop engine function, while a car is stuck in traffic congestion, the car engine will automatically shut down, and then restart again when the car moves on as the traffic congestion clears. When the car engine restarts, a sudden high in-rush current is drawn from the car's battery, causing a rapid plunge of the battery's voltage. After the car engine has restarted, the battery returns to normal operational values. The supply voltage of a car entertainment system is derived directly from the car's battery and thus experiences a same voltage variation for the whole start-stop duration.
Usually, when the car engine is in a starting state, the capacitor of an external circuit that establishes a quiescent operating point has a fast discharge behavior. This will cause the power amplifier of a car radio system to no longer work properly. The car radio amplifier will then be set to a mute state so that no sound (including transient noise from a loudspeaker) can be heard.
Today, people are seeking a more comfortable driving experience and thus have much higher requirements for a car entertainment system than before. The interruption of the audio output is unacceptable and thus the current sound quality standard on cars does not allow this behavior even in a starting state.
In order to avoid the above mentioned problem, a well known external circuit solution is used in which a further DC (direct current)/DC regulator is used to stabilize the battery voltage during the start-stop. However, this solution requires several peripheral devices and bulky LC components, and thus causes an increased total cost, board dimension and system complexity.
Another solution has been proposed and adopted in cars, in which the minimum work supply voltage is set at a half of the power supply voltage Vcc (e.g. a half of the battery's supply voltage, for example, if the battery voltage is 12V, then Vcc/2=12V/2=6V). The typical circuit structure of this solution is shown in FIG. 1, which comprises a clamping circuit M1, a class AB amplifier M2, and a common feedback (CMFB) circuit M3. In the circuit as shown in FIG. 1, the clamping circuit M1 is configured to provide a clamped voltage Vsvr=Vcc/4 at a SVR (Supply Voltage Rejection) node to the class AB amplifier M2 as its input bias voltage. The class AB amplifier is a typical circuit structure used in a car radio system for driving a loudspeaker and usually comprises a pair of amplifiers, such as operational amplifiers, and four feedback resistors Rf1, Rf2, Re1 and Re2, which are typically coupled between respective outputs and inputs of the pair of amplifiers. Normally, resistances of resistors Rf1 and Rf2 are set equal and 20 times of the resistances of resistors Re1 and Re2, i.e. Rf1=Rf2=20Re1=20Re2. The common mode feedback circuit M3 is interposed between the outputs of the pair of amplifiers and corresponding inputs thereof to absorb the quiescent current from the output to the input. With this CMFB, the outputs of the pair of amplifiers can be kept at Vcc/2 and the alternating current (AC) gain of outputs OUTP and OUTM of the pair of amplifiers can be maintained equal. In this solution, the inputs to both amplifiers are biased at one fourth of the power supply (i.e. Vcc/4), while the outputs of the two amplifiers are biased at the half power supply (i.e. Vcc/2), so that when the power supply drops from Vcc to Vcc/2 in a starting state, the outputs of the amplifiers are reduced proportionally to the reduction of the power supply and thus the whole amplifier can still operate normally.
However, although the solution with the CMFB can solve the start-stop problem, it additionally introduces a positive feedback loop which may greatly affects the stability of the whole radio system especially when a car is powered up. For example, if the external load is a capacitive load, e.g. with a capacitance of 10 nF, this circuit may cause an unwanted oscillation. In the actual application, the external capacitance cannot be avoided. For example, for a loudspeaker with a resistance of 2 ohm the oscillation may occur unless the capacitance induced by the loudspeaker is less than 2 nF, but it is not practical.